Chemical sensors may be based on crystalline domains of colloidal particles embedded in a gelled matrix. Appropriately prepared gels, such as hydrogels, can swell and contract in response to changes in their physical or chemical environment. The embedded colloidal crystal's lattice constant changes as the gel swells, and its optical diffraction properties vary accordingly, in a manner that may be detected. This response can be optimized for a particular stimulus by introducing functional groups, into the gel capable of interacting with the stimulus and inducing the desired structural changes to the gel.
Implementations of this technique have incorporated the necessary functional groups directly into the polymer from which the gel is constituted (see Asher, U.S. Pat. No. 6,544,800, and Asher et al., U.S. Pat. Nos. 5,187,599, 5,854,078, and 5,898,004—hereafter the “Asher patents”). Once a gel is functionalized in this way, it swells preferentially in response to the specific stimulus, and the embedded colloidal particles are used as passive tracers of the mechanical state or degree of swelling of the functionalized hydrogels. As described in the Asher patents, the gels' swelling is monitored by measuring the optical properties of a colloidal crystal embedded within the gel. Variations in lattice constant change the diffraction pattern of the colloidal crystal in a way which can be measured.
In particular, as described in the Asher patents, uniformly sized colloidal spheres were allowed to organize themselves into a self-assembled crystal, and then the hydrogel, incorporating a concentration of selected functional groups, was polymerized around the as-formed crystal. Once formed, the functionalized gelled crystal can be used as quantitative sensors for the stimulus to which the functional groups respond by monitoring their diffraction using an optical spectrometer. Incorporating different functional groups into different gels results in a variety of sensor materials sensitive to various specific environmental stimuli, each gel sensitive to a specific stimulus.
However, the above approach by the Asher patents are only effective for detecting one stimulus at a time, and they cannot be scaled up easily to monitor a spectrum of potential stimuli. Further, combining several sensor crystals into a single system capable of detecting and distinguishing different stimuli poses significant practical difficulties, would require post-synthesis assembly, which is a difficult process. Further, no method exists for synthesizing distinctly functionalized regions of gel in close proximity.
Still further, allowing colloidal spheres to self-assemble into a colloidal crystal and then creating a gel around the resulting lattice as described in the Asher patents, introduces undesirable variability into the optical properties of the resulting sensor, both because a self-assembled colloidal crystal suffers from structural defects, and also because very little control is possible over the crystal lattice constant, and still less over its symmetry.
Finally, if the individual self-assembled crystal had indistinguishable or unpredictable optical properties, then detecting the different responses to different stimuli in a sensor array would require position-sensitive detection, which is costly. Having sensor domains with distinctive optical signatures would be less costly and difficult, but achieving such control during colloidal crystallization is extremely difficult, and may be impossible.
Thus, a broad spectrum sensor that can detect and monitor large numbers of disparate environmental stimuli simultaneously, and that is easy and inexpensive to manufacture, is needed.